Field of the Invention
The present invention relates to an imaging apparatus, an imaging system, and a driving method of an imaging apparatus.
Description of the Related Art
When an imaging apparatus is irradiated with very strong light such as solar light, if correlated double sampling (CDS) is performed on a pixel irradiated with the strong light, a signal level may decrease and an image darker than original brightness may be generated. This phenomenon will be called a high-brightness darkening phenomenon.
In an imaging apparatus of Japanese Patent Application Laid-Open No. 2009-194569, a clipping unit is electrically connected to a vertical signal line to which a pixel outputs a signal. When a noise-level signal is output to the vertical signal line, the clipping unit described in Japanese Patent Application Laid-Open No. 2009-194569 controls an electric potential of the vertical signal line so as not to come closer to an optical signal-level electric potential than a predetermined electric potential. The imaging apparatus of Japanese Patent Application Laid-Open No. 2009-194569 thereby suppresses generation of the high-brightness darkening phenomenon. The clipping unit includes a transistor having a source and a drain, one of which is electrically connected to a power supply line. The other one of the source and the drain is electrically connected to the vertical signal line. A first node of a holding capacitance is electrically connected to the gate of this transistor. In addition, a second node of the holding capacitance is electrically connected to a shift unit serving as an electric potential supply unit. Japanese Patent Application Laid-Open No. 2009-194569 further describes a configuration in which an electric potential of the first node of the holding capacitance is shifted by the shift unit shifting an electric potential of the second node of the holding capacitance.
In the imaging apparatus of Japanese Patent Application Laid-Open No. 2009-194569, a signal holding unit connected to the vertical signal line resets the electric potential of the second node of the holding capacitance during a period from the end of sampling of a noise-level signal output to the vertical signal line to the end of sampling of an optical signal output to the vertical signal line. Through this reset operation, a charge/discharge current flows between the power supply line electrically connected to the holding capacitance and the holding capacitance. This charge/discharge current generates electric potential fluctuations in a power supply line of the electric potential supply unit. Such electric potential fluctuations in the power supply line generate, when the power supply line of the electric potential supply unit is electrically connected to a power supply line of a pixel, noise in a signal output from the pixel. The noise decreases the quality of an image generated according to a signal output from the imaging apparatus. Thus, in some cases, the charge/discharge current that flows from the holding capacitance to the power supply line electrically connected to the holding capacitance may generate noise in an optical signal output from a pixel.